OTEC (ocean thermal energy conversion)
by Chris Woodford. Last updated: February 23, 2023.
Earth? It might be better called Oceanus: most of
it is, after all, covered in water—much of it very warm
water. The really interesting thing about the ocean is not how hot it
is, but the difference in temperature between the surface (where the
Sun keeps the sea relatively hot) and the depths (where the water, never warmed by the Sun, is
considerably cooler). As any engineer knows, a temperature difference
like this is very useful indeed if you're trying to make power. So
why not use the heat in Earth's vast oceans to generate useful energy? That's
the basic thinking behind OTEC
(ocean thermal energy conversion), first suggested in 1881, which
involves extracting useful energy from the heat locked in the oceans.
How much energy are we talking about? According to some estimates,
there's enough heat in the upper layers of the oceans to meet
humankind's energy needs hundreds of times over.
[1]
Sounds great! So... how exactly does it work? Let's take a closer look!
Artwork: The concept of OTEC: could we really use heat differences in the ocean (left) to make electric power for things like laptops and kettles?
What is OTEC?
Most of the electricity we use comes from heat
engines of one kind or another.
A heat engine is a machine that cycles between two different temperatures, one hot and one cold,
usually extracting heat energy from a fuel of some kind. In a
steam engine
or a steam turbine, for example, coal heats water to make hot,
high-pressure steam, which is then allowed to expand and cool down to
a lower temperature and pressure, pushing a piston and turning a wheel as it does so. The
greater the temperature difference between the hot
steam and the cooled water vapor it becomes, the more energy can be
extracted (and the more efficient the engine).
Artwork: Temperature gradients: As you can see from this NASA map of ocean temperatures, there are huge variations in ocean temperature between warm tropical areas (red, orange, and yellow) and colder polar and temperate regions (green and blue). What you can't see from this map is the variations in temperature that exist at different depths of the ocean in the same region. The tropics (the area colored orange and yellow) have the best potential for generating OTEC power. Image by NASA MODIS Ocean Group, Goddard Space Flight Center, and the University of Miami courtesy of
NASA Goddard Space Flight Center (NASA-GSFC) and
Internet Archive.
In OTEC, we use the temperature difference between
the hot surface of the ocean and the cooler, deeper layers beneath to
drive a heat engine in a broadly similar way—except that no fuel is burned:
we don't need to create a difference in temperature by burning fuel because
a temperature gradient exists in the oceans naturally! Since the temperature difference is all-important, we need the biggest vertical, temperature gradient we can possibly find (at least 20° and
ideally more like 30–40°). In practice, that means a place
where the surface waters are as hot as we can find and the deep
waters (perhaps 500–1000m or 1000–3000ft beneath ) are as cold as
possible. The best place to find such a combination is in the tropics
(between the latitudes of about 20°N and 20°S).
Chart: How ocean temperature various with depth. In the warmest, tropical parts of the world's oceans, surface temperatures are typically 20°C (68°F) or more. In the coldest depths, they're close to freezing (around 4°C or 39°F). This huge temperature difference makes OTEC possible. Water temperature changes rapidly with depth in the middle region, which is known as the thermocline.
How much power could OTEC make?
“... tropical seas absorb an amount of solar radiation equal in heat content to about 250 billion barrels of oil.”
NREL
Considering how big and deep the oceans are, it comes as no surprise to find they
soak up and retain vast amounts of solar energy. Some years ago
ocean engineer Richard Seymour estimated that the oceans
and atmosphere between them "intercept... about 80 trillion kW, or
about one thousand times as much energy as used... globally."
[2]
How much of that could we recover from the sea?
According to the US Department of Energy's National Renewable Energy Laboratory
(DOE/NREL), on a typical day, the tropical oceans mop up heat energy
equivalent to 250 billion barrels of oil. Converting a mere 0.005
percent of this into electricity would be enough to power the whole
of the United States! However, impressive-sounding estimates like
this don't take account of the tremendous practical difficulties
involved in harvesting ocean energy.
How does OTEC work?
There are essentially two different kinds of OTEC plant, known as closed cycle and open cycle.
Closed cycle
In closed-cycle OTEC, there is a long, closed loop
of pipeline filled with a fluid such as ammonia, which has a
very low-boiling point (−33°C or 28°F).
(Other fluids, including propane and various low-boiling refrigerant chemicals, have also been successfully used for transporting heat in OTEC plants.) The ammonia never leaves the pipe: it simply
cycles around the loop again and again, picking up heat from the
ocean, giving it up to the OTEC power plant, and returning as a
cooled fluid to collect some more.
How does it work? First, the pipe flows through a
heat exchanger fixed in the hot surface waters of the ocean, which
makes the ammonia boil and vaporize. The heated ammonia vapor expands
and blows through a turbine, which extracts some of its energy,
driving a generator to produce electricity. Once the ammonia has
expanded, it passes through a second heat exchanger, where cool water
pumped up from the ocean depths condenses it back to a liquid so it
can be recycled. You can think of the ammonia working in a broadly
similar way to the coolant in a refrigerator, which is also designed
to pick up heat from one place (the chiller cabinet) and carry it elsewhere
(the room outside) using a closed-loop cycle. In OTEC, the ammonia
picks up heat from the hot, surface ocean waters (just as the coolant chemical
picks up heat from the chiller compartment), carries it to a turbine where much of its energy is extracted, and
is then condensed back to a liquid so it can run round the loop for
more heat (just as the coolant in a refrigerator is compressed and
cooled in the fins around the back of the machine).
Open cycle
Photo: A model of a simple open-cycle OTEC system. The heart of it is a large turbine driven by
steam, which is cooled by water pumped up from the deep ocean. Photo by Warren Gretz
courtesy of US DOE/NREL.
In open-cycle OTEC, the sea water is itself
used to generate heat without any kind of intermediate fluid. At the
surface of the ocean, hot sea water is turned to steam by
reducing its pressure (remember that a liquid can be made to
change state, into a gas,
either by increasing its temperature or reducing its pressure). The
steam drives a turbine and generates electricity (as in
closed-cycle OTEC), before being condensed back to water using cold
water piped up from the ocean depths.
One of the very interesting byproducts of this method is that heating and condensing sea water
removes its salt and other impurities, so the water that leaves the
OTEC plant is pure and salt-free. That means open-cycle OTEC plants
can double-up as desalination plants,
purifying water either for drinking supplies or for irrigating crops. That's a
very useful added benefit in hot, tropical countries that may be short of freshwater.
Land- and sea-based OTEC
Open- and closed-cycle OTEC can operate either on
the shore (land-based) or out at sea (sometimes known as floating or
grazing). Both have advantages and disadvantages, which we'll
consider in a moment. Land-based OTEC plants are constructed on the
shoreline with four large hot and cold pipelines dipping down into
the sea: a hot water input, a hot water output, a cold-water input,
and a cold-water output. Unfortunately, shoreline construction makes
them more susceptible to problems like coastal erosion and damage
from hurricanes and other storms.
Photo: Coming in to land: Land-based OTEC means you have to pipe huge volumes of water ashore,
which is what this giant cold-water pipeline does at the Natural Energy Laboratory at Keahole Point, Hawaii.
Sea-based OTEC avoids this problem, but you have to carry the electricity generated offshore to land instead. Photo by Warren Gretz courtesy of US DOE/NREL.
Sea-based OTEC plants are essentially the same but have
to be constructed on some sort of tethered, floating platform, not
unlike a floating oil platform, with the four pipes running down into
the sea; early prototypes were run from converted oil tankers and barges. They also need a cable running back to land to send the
electrical power they generate ashore. Hybrid forms of OTEC are also
possible. So, for example, you could build an OTEC platform some
distance offshore on the continental shelf, which would share some of
the advantages of land-based OTEC (stability and durability,
closeness to the shore, and so on) and floating OTEC (opportunity to
exploit a greater temperature gradient, so generating power more
efficiently).
Advantages and disadvantages
Advantages
OTEC sounds immensely attractive: it's clean,
green renewable energy that doesn't involve burning fossil fuels,
producing large amounts of greenhouse gases, or releasing toxic
air pollution. By helping to reduce our dependence on fuels such as
petroleum, OTEC could also help to reduce the "collateral" damage the world suffers from an oil-dependent economy—including wars
fought over oil and water pollution from tanker spills. It could also provide a very
useful source of power for tropical island states that lack their own
energy resources, effectively making them self-sufficient. As we've
already considered, open-cycle OTEC can play a useful part in
providing pure, usable water from ocean water. OTEC can also be used
to produce fuels such as hydrogen; the electricity it generates can
be used to power an electrolysis plant that would split seawater into
hydrogen and oxygen, which could be bottled or piped ashore and then
used to power such things as fuel cells
in electric cars. The waste cooling water
used by an OTEC plant can also be used for aquaculture
(growing fish and other marine food such as algae under controlled
conditions), refrigeration, and air conditioning.
Disadvantages
The biggest problem with OTEC is that it's
relatively inefficient. The laws of physics (in this case, the
Carnot cycle) say that any practical heat engine must operate at less than 100 percent efficiency; most operate well below—and OTEC plants, which use a relatively small temperature difference between their hot
and cold fluids, have among the lowest efficiency of all: typically
just a few percent.
[3]
Taken by itself, low efficiency doesn't matter
too much because the energy in ocean water is completely free:
if we capture only a small proportion of what's there, it's not a huge problem.
However, low efficiency does have other consequences.
OTEC plants have to work very hard (pump huge amounts of water) to produce even modest amounts of
electricity, which brings two problems.
[4]
First, it means a significant amount of the electricity generated (typically about a third) has to
be used for operating the system (pumping the water in and out).
Second, it implies that OTEC plants have to be constructed on a relatively
large scale, which makes them expensive investments. Large-scale
onshore OTEC plants could have a considerable environmental impact on
shorelines, which are often home to fragile, already threatened
ecosystems such as mangroves and
coral reefs.
Photo: Onshore OTEC plants can take up a lot of valuable coastal land. This is the Natural Energy Laboratory at Keahole Point, Hawaii. Photo by Warren Gretz courtesy of US DOE/NREL.
Although OTEC plants are only suitable for
tropical seas with relatively large temperature gradients, that's
less of a problem than it sounds. According to
DOE/NREL, OTEC could
theoretically operate in 29 different sovereign territories
(including warmer, southern parts of the United States) and 66
developing nations; and temperate parts of the world that can't
operate OTEC most likely have alternative forms of ocean power they
could exploit, including offshore wind turbines, tidal barrages, and wave power.
Although OTEC produces no chemical pollution, it
does involve a human intervention in the temperature balance of the
sea, which could have localized environmental impacts that would need
to be assessed. One important (and often overlooked) impact of OTEC
is that pumping cold water from the deep ocean to the surfaces
releases carbon dioxide, the greenhouse gas currently most
responsible for global warming. The amount released is only a
fraction (perhaps 10 percent) as much as that produced by a
fossil-fueled power plant, however.
[5]
How far off is OTEC?
Scientists and engineers have been trying to
extract useful heat energy from the oceans for over a century, with
varying amounts of success. So far, only a few small-scale
experimental units are operating. One is producing about 100kW of
electricity (about 5–10 percent as much as a single 1–2MW wind turbine) in
Japan, another is generating about half as much in Hawaii, and
a third is now producing about 1MW in India; these are tiny amounts of energy that don't prove the
long-term commercial viability of OTEC in a world where there are
many other sources of power, other forms of renewable energy
(such as wind and solar) are becoming dramatically cheaper, and the economics of energy have to
be rewritten from one day to the next.
All that could be about to change, however. After years of planning and construction, the Lockheed Martin company finally finished work on its 100kW prototype OTEC plant in Hawaii in August 2015;
work began on the Global Ocean reSource and Energy Association Institute's equally tiny 100kW
OTEC demonstration facility in Kumejima, Okinawa in 2013. Depending on how successful these
modest experiments prove to be, bigger plants could follow; Lockheed has already announced plans for a 10MW offshore plant (with 100 times more generating capacity) in China,
while KRISO (the Korean Research Institute of Ships and Ocean Engineering) has been developing a 1MW OTEC
unit for the Pacific island of Kiribati since 2013.
Under current economic conditions, OTEC plants are most likely to be constructed in or near small tropical islands like this that have little or no energy resources of their own,
a high-dependence on expensive, imported oil, and perhaps a pressing shortage of freshwater as well; a combined OTEC power and desalination plant could be very attractive in that
situation. Early customers are likely to include power-hungry US
naval bases in tropical American territories—and that's one of the
reasons why the US Navy is currently investing in the technology.
All this sounds very exciting, but OTEC is still, essentially, a prototype
technology that's harder to perfect in practice than to conceive in theory.
In 2018, for example, amibitious plans to construct a 16MW offshore OTEC plant in Martinique were shelved
indefinitely following major technical difficulties with the coldwater inlet pipe.
Barbados and
Puerto Rico
are among the other nations still actively investigating OTEC as a future power source.
Who invented OTEC?
Here's a brief timeline of some key moments in the
history of ocean thermal energy.
- 1881: French physicist Jacques d'Arsonval suggests extracting heat energy from the oceans.
- 1926: Georges Claude, a student of d'Arsonval's, builds a prototype, on-shore energy-extracting machine on the coast of Cuba. In 1935, he tries and fails to construct an experimental off-shore OTEC plant on a cargo ship. With Paul Boucheret, Claude receives a US patent for an open-cycle OTEC system (number 2006985) on July 2, 1935.
- 1927: OTEC gains first widespread publicity when Albert G. Ingalls writes up the idea in an article "Inexhaustible Power from Sea Water—a Dream or a Prophecy?" in Scientific American (May 1927, pages 339–342).
- 1960s: American engineer J. Hilbert Anderson (a specialist in refrigeration and heat cycles) and his son James Anderson, Jr. begin studying ocean thermal energy. Having identified major shortcomings in Claude's OTEC plant, they propose using a closed loop of "working fluid" to remove heat from the upper ocean in a similar way to the mechanism of a refrigerator. They're granted US patent 3312054 for their "Sea Water Power Plant," based on closed-cycle OTEC using propane as the working fluid, on April 4, 1967.
- 1974: The United States opens the Natural Energy Laboratory of Hawaii (NELHA)
on 130 hectares (322 acres) of land at Keahole Point on the Kona coast as its primary test laboratory for OTEC. Using closed-cycle technology, it successfully builds a prototype, offshore, "mini-OTEC" plant on a US Navy barge.
- 1980: India begins a long series of research studies into OTEC, currently led by its National Institute of Ocean Technology (NIOT).
- 1982: Tokyo Electric Power Company and Toshiba successfully construct a small (100kW) OTEC plant on the island of Nauru, though much of the electricity is used to operate the plant and only 30-40kW is successfully fed into the power grid.
- 1993: The Natural Energy Laboratory sets a new record for open-cycle OTEC of 50kW. Six years later, it successfully tests a 120kW closed-cycle plant.
- 2008: Tamil Nadu Electricity Board is operating an experimental 1MW plant at Kulasekarapattinam, near Tiruchendur in the Tuticorin district.
- 2009: US Navy contracts Lockheed Martin to develop a 5–10MW OTEC plant (currently budgeted at $12.5million).
- 2013: KRISO (Korean Research Institute of Ships and Ocean Engineering) and the government of Kiribati begin to collaborate on OTEC projects, initially deploying test equipment on ocean barges.
- 2015: Lockheed Martin opens its OTEC plant in Hawaii, connects it to the US power grid, and announces
plans for a much more ambitious 10MW plant in China.
- 2018: The 16MW NEMO (New Energy for Martinique and Overseas) offshore OTEC project is abandoned following the discovery of major technical problems.
- 2019: British company Global OTEC Resources proposes small floating barges for harnessing OTEC in tropical developing nations.
- 2020–2021: KRISO plans to deploy a 1MW land-based OTEC plant at South Tarawa, Kiribati.
